fix(federation): Require user-defined @tag directive definition when @tag is used

[trevor-scheer]

• Document new error codes

[glasser]

Could this instead be structured as "require all used directives to be defined, with exceptions made for historical reasons for the Federation Spec directives"?

[trevor-scheer]

Some good discussion spun out of @glasser's comment that resulted in us realizing a great way to do typical SDL validation of the subgraphs. We can perform most of the validations and just omit the rules that make a subgraph invalid + filter against our special federation directives.

A snippet, which I'll capture in a separate issue as well.

import { federationDirectives } from '../../../directives';
import { specifiedSDLRules } from 'graphql/validation/specifiedRules';
import { validateSDL } from 'graphql/validation/validate';
import { ServiceDefinition } from '../../types';
// import { errorWithCode, logDirective } from '../../utils';

const rulesToExclude = ['KnownTypeNamesRule', 'PossibleTypeExtensionsRule'];
const errorsToFilter = federationDirectives.map(directive => directive.name);
/**
 * If there are tag usages in the service definition, check that the tag directive
 * definition is included and correct.
 */
export const graphqlValidations = ({
  name: _serviceName,
  typeDefs,
}: ServiceDefinition) => {
  const filteredRules = specifiedSDLRules.filter((rule) =>
    !rulesToExclude.includes(rule.name),
  );
  const errors = validateSDL(typeDefs, undefined, filteredRules);
  return errors.filter(({ message }) => {
    return !errorsToFilter.some(keyWord => message.includes(keyWord));
  });
};

Going to apply a similar approach (using graphql-js and specific rules) to accomplish some of this instead.

[glasser]

I don't necessarily think we need to add ad hoc "defined correctly" checks before we have a solid pattern but it doesn't hurt. I have a few suggestions below but otherwise looks good.